Considering the resources conservation and environmental issues, it is required for a sliding member, such as an engine piston or a component used for valve system to reduce frictionally-induced energy losses. For this reason, various surface treatments have been conducted hitherto on the sliding surface of the sliding member to reduce the value of coefficient of friction thereof or to improve wear resistance. Among them, an amorphous hard carbon film called diamond-like carbon (hereinafter called, DLC) is widely used as a film which can improve sliding property on the sliding surface.
In Patent Publication 1 (JP2005-336456 A), a low friction sliding member is disclosed, which includes DLC-Si (Diamond-like carbon containing Silicon) film. Such DLC-Si film, when applied under a wet oiling condition that lubricant oil containing additive agent, such as engine oil, is used can not sufficiently express low friction property. However, when water exists in the engine oil, an adsorbed water layer is formed on the DLC-Si film via silanol layer (Si—OH layer). This adsorbed water layer reduces the coefficient of friction.
Another sliding member is developed, which includes a DLC film containing a metal element instead of containing silicon. For example, in Patent Documents 2, 3 and 4, a (listed below) sliding member which includes a DLC film containing Ti (titanium), Cr (chromium), Mn (manganese), Fe (iron), Ni (nickel), Zn (zinc), Ga (gallium), Nb (niobium), Mo (molybdenum) or W (tungsten) is disclosed. The sliding member including a DLC film forms an additive layer on the surface of the DLC film to achieve low coefficient of friction by sliding in the lubricant oil containing such additive.